204 PLANT LIFE. 



extends to all parts of the tendril, which begins to curve. 

 As both ends are fast, it is a mechanical necessity that the 

 curves become spiral coils, both right- and left-handed, ac- 

 companied by a twisting of the tendril on its axis ( fig. 107). 

 After the coils are formed the tissues of the tendril become 

 thick-walled and rigid, so that the plant is attached to the 

 support by a series of spiral springs. 



Other tendrils do not nutate, but are negatively heliotropic, 

 and by contact their tips are stimulated to develop disks 

 which apply themselves closely to the support, and send into 

 its irregularities short outgrowths from the surface cells. 

 Such plants are adapted to support themselves by walls, tree- 

 trunks, etc. The Japanese ivy and one form of the Virginia 

 creeper are notable examples. 



The coiling of the leaf-stalks is not unlike the first curva- 

 tures described for tendrils (fig. 154). 



294. (B) Movements of turgor.— The movements just 

 described are confined to members which are growing either 

 throughout or in some part. As turgor can affect only tis- 

 sues whose cell-walls are elastic (*j 188), the movements 

 produced directly by variation in turgor can occur in such 

 mature members only as are provided with special motor 

 organs. In almost all cases these are leaves. Stimuli which 

 regulate growth (^[ 284) may also affect motor organs, pro- 

 ducing like curvatures. But elongation of any part of a motor 

 organ by increased turgor is reversible, not permanent, (cf. 



II 254)- 



295. Motor organs. — The motor organ in leaves is usually 

 the leaf base (^[ 151) or a modified portion of the petiole, 

 sometimes greater but generally less in diameter than the 

 rest. Its cortex consists of large, rather thick-walled, pa- 

 renchyma cells, and the stele occupies a relatively small part 

 of the transverse section. In other parts of the petiole the 

 stele is much larger, or there may be several steles distributed 



